Process for the manufacture of cryolite



- discussionofrthe steps \This invention relates to a process for the manufacture of cryolite. More particularly; thisinventionWelatcs' -to an eflicient and economical-process for the manufacture of silica-free cryolite. (suitable for use in the electrolytic manufacture of aluminum) employin'g'as the primary raw materiala 'by-product from th'erproccssing :bauxite'.:of;oreswhich has no present industrial utility. t

-iMany bauxite anclbauxiti'cfores foundthr'oughout' the i world (such as the extensive depositszmined an-Arkansas) contain: smallamonnt-s of. fluoride- (probably bound as calcium fiuoride) and sulfide".:(probably bound as iron pyrites)'. These elements gfind their-way into the' red mu r. (the 'desilication residue) of the-well-knownrBayer process'for the recoveryof aluminalfrom-bauxite.- 1It,-is

United States oration of a As a source of'the (aluminum oxide) derived from. any :source whatever may;

I l sten d Feb. 16,1960

actionvwith the double salt and the alumina'or-aluminum hydroxide may be effected at any temperatureifromroom: temperature to 130 C. (at which-temperature HBFriboilsz with decomposition); The reaction may beefiectedat; s'ubat'mospheric, atmospheric or superatmospheric pres sure. i v I, 4

aluminum :in the cryolite aluminaf be:used(such as the calcined alumina from the'Bayer; process or the cornbined process; However,- .'because=of its::greater reactivity I- prefer to use;.a1uminum hydroxide and preferablythewashed, wet filter cake ofaluminum" hydroxide obtained in the Bayer process, prior'to calciriation'rzto alumina. y

=-Foryoptimum reagent economy and yields, the-.double sa'ltg; aluminum oxide or (hydroxide, and ,fluoboric .acid

. solution should be us'edin stoichiometric proportions, Le.

fourrmoles of double salt, five moles of fluoboric acid, and four.mole.-equi,valents of-aluminum oxide or.aluminum-' hydroxide. The cryolite formed is filtered ofi;.washed= with .cold Water" and 'thecombined. filtrate and washings 1' (containing 'boric-iacid and sulfuric acid) is used in the second step ?of the process'to obta'inthe fluoboricacid required in the v firs'tast ep of the process.-

' preferlto; .effectxthereaction by digesting the. double I salt,;the' aluminum oxide-or hydroxide," and the ,fiuoboric acid at'i90? C. to 1100", C. for; one to twohours, with goodnow-common practice in' manyiplants' to recover alumina and-soda values from this'red':mudresidue by-sinterin'g theisarne withllimestonezand soda ash (the so-called -combinat'ion processl or soda-limesintr process?) and leaehe ing sodium aluininate froml'the'sinter, During .the'calcinatiorrofthei red mud in this process, :the pyritesare probably oxidized toironsulfatest The sulfate ion in 'the I latter andxlthexfluoride in th'e caF are solubili'zed by'the soda-inlthe kiln'feed, and are-dissolved outtof-the sinter with the isbdium aluminate leachliquors; After aluminarecovery from :these leach liquors, therspentliquors are concentrated for-recycling tothe-processi. During. the concentration, the sulfate and.'fiuorideprecipitatesoutlasa doubletsalt ofv composition Na SO qNaF; whichis filtered olinnd-rccovered. About 1-3'.'5"to14.0.-lbs;* of thisidouble saltuare'; recovered per tonccf Arkansas bauxite-sore Processed. a a -Atp'resent;thisdouble salt;byeproduct has no industrial useiaud is'larg'ely'being discardcd. -'It is =the purpose-of this invention to provide a process forj theznianufa'cture ofsilica-free cry'olite wherebyall gofathe alkali pvalue of this double saltVNa SO -NaF:a'n'd. all; ofethe fluoride-value utilized to replacegand:economize'wercostzofg an equivalent ambunt ofsulfuric acid in the; supplying of :the' deficit 'of fluoride requiredto form NagAlF i a :Thisainvention may. best-beiunderstood} by: a-seriatimafirst step of this-process involves the re'actionof the double s al-tt. (NagSO NaF with ran-aqueous rsolutionwideranges. Excellent yields agitation, and thereafter filtering ofi theNa A1F precipitate from-thesolutionof 'boric acid-and sulfuric acid; The:

cryolitefiis washed with-a little water'and dried. -.These:.-.

conditions. are'byno means criticaland mayv bevaried over of cryolite are obtainedby this procedure;

' r' I STEP II In the secondstep of'this process,vthe' filtrate and washings of boric acid and sulfuricacid obtained in the first;

step are digestedwith a 'calcium-fluoride-containing,material and sufiicient additional sulfuric acid, to regenerate the aqueous solution containingVS moles of HBF required in thefirststep of this process,-accordiug tothe equation:

- are recovered ascryolite and all of the sulfate' 'value is I The formation of fiuoboric acidby the reaction of ,calci- I um fluoride in the presence of sulfuric acid and boric acid is well. known (Heiser, Chemical Engineering Progress 45, #3, 169-179 (1949);. US. Patents 2,182,509-511 1939). This maybeeffected by:the digestion of a calciurn fluoride-containing material with, sulfuric acid and boric' acid inaqueous solution between room temperature and C.,.at subatmospheric, atmospheric orsuperatmos'ph eric pressures, I I I I I The-calciumfluoride-containing material is .preferably fluospar (also knownas fiuorite, fluor and (in GreatBrit- I ain) as 'Derbyshire spar. Although acid-grade fiuorspar containingza minimum of '95 Cal' is preferred foruse inlthis process,much poorer grades of fluorspar maybe,

of cfluoboric acid in the presence ofaquantitypf'alumi num oxide or aluminum hydroxideg zaccording tothe'- I Thesolu tioniof anemic acidLWhichi is' derived ill the i second"s'tep of this process, "may contain; considerable amounts of free. boricracid. It may, range in' NI -I BE, conemrauen from et-1 w as magen a; nasal-1 "m c out correspondingi I- lBFr formation. ,However, aghigha. silicarfluorspar may be usedalmost entirely wi thoutref- M daylcryclite-pr'ofcessew a, v I

used, since it is an inherent advantageof this process that the silica content of thefluorsparldoes not interfere-with the formation of HBF but remains behind as arr-insoluble precipitate 'whic h i s fi l t ered- 01ft"; it I is feasible; to use fluorspars containingras'littleas-60% CaF It is desirable,

to-use fluorspars containing; as, littleCaCO; as possible since this: component only. consumes sulfuric acid r-wither'ence to its silica content ifits .CaCOyislow, 'lihi snperd ri of this" invention,-; ;wliic s not tfeasible withkmany present The fluoboricacid formation is effected by digesting the fluorspar in an aqueous medium with sulfuric acid and boric acid. for optimum yields the sulfuric acid is used in' amounts corresponding to that theoretically refiuoboric acid and the lowest silica retention in-the HBF; solution have been obtained by employing boric acid in amountsequivalen-t. to 120% to 150% of the amount theoretically required for HBF formation. Since this excess of boric acid simply recycles in the process, without loss, the use of this stoichiometric excess in no way adds to the cost of the process.

The digestion of the fluorspar, sulfuric acid and boric acid in solution is effected for a period of time suflicient to convert all of the fluoride in solution to HBF usually from 2 to'8 hours at 80 C. to 100 C. At the conclusion of this period, the calcium sulfate is filtered from the solution of fluoboric acid, the filtercake is Washed with a little hot water and the filtrate and washings are combined for fiuoboric acid recovery.

In the process of my invention, the filtrate from the first step (containing at least 5 moles of boric acid, and preferably from 120% to 150% as much (i.e. 6 to 7.5 moles of boric acid) and 4 moles of sulfuric acid) is mixed with sutficient finely ground fluorspar to give the reactive equivalent of 10 moles of CaF and suflicient additional sulfuric acid to give the reactive equivalent of the six additional moles of H 80 required by theory (see the above equation) and sufficient sulfuric acid to neutralize the CaCO present in the fluorspar. This reaction may be: effected over a wide range of reagent concentration, at subatmospheric, atmospheric and superutmospheric pressures, at temperatures between room temperature and 130 C. I prefer to effect this digestion of the fluorspar at temperatures between 80 C. and 100" C. for periods of time ranging from two to eight hours. At the conclusion of this period, the (laSO, is filtered off, washed with a little hot water, and the filtrate and washings (containing 5 moles of HBE, and (optionally) 1.0 to 2.5 moles of free boric acid) are recycled to the first step of this process. The CaSO precipitate will retain all of the silica originally present in the fluorspar.

The following example is given to define and to illustrate this invention, but in no way to limit it to reagents, proportions or conditions described therein. Obvious modifications will occur to any person skilled in the art.

Example and 124 gms. of boric acid (2 moles).

ings are combined and the Washed Na AlF is dried by the processes well known in the art.

The filtrate and washings (about 4.5 liters) containing 392 gms. H SO (4 moles) and 433 gms. of boric acid (7 moles) is now mixed with 971 gms. of a finely ground fluorspar (analyzing 84.5% CaF 2.8% CaCO and 12.7% SiO and 718 grns. of 66 B. sulfuric acid (667 gms. H 50 or 6.81 moles). The fluorspar added contains the equivalent of 821 gms. CaF (10.5 moles) and 27 gms. CaCO (0.3 mole). The total amount of sulfuric acid in the solution (10.81 moles) is suficient to react with all'of the CaCO and CaF present. The reaction mixture is digested at 90 C. to 95 C., with little hot water, and the filtrate and washings are combined. There is thus obtained about four liters of a solution containing 440 gms. of fiuoboric acid"(5v moles) This solution is recycled to the first step of the process.

On recycling of motherliquors, the materials balance of this process may be given as follows: 1

736 parts of double salt (Na SO -NaF) 312 parts of aluminum 'hydroxide'(or 204 parts of alumina) 971 parts of fluorspar (analyzing"84.5% CaF 2.8% CaCO and 12.7% SiO (or equivalent of 821 parts of CaF 718 parts of 66-B. sulfuric acid Yield 810 to 820 parts of cryolite (Na AlF and a residue of calciumsulfate and silica which vis dis-;

carded.

The cryolite made by this process analyzes 99.92% Na AlF with less than 0.04% SiO and less than 0.02% Fe O It must be emphasized that this process, unlike any process of the prior art, does not require caustic soda or soda ash as a primary raw material. All of the. sodium in the cryolite is supplied as the double salt (Na SO -NaF). Similarly one-sixth of the fluorine in the final cryolite is supplied by the double salt. Finally 40% of the sulfuric acid used in the process for the preparation of the fluoboric acid is derived by the utilization of the sulfate ion in the double salt. Thus, we derive a complete utilization of a product which is obtained in appreciable amounts as a by-product in bauxite proc' essing and which has never been usedindustrially before.

On the basis of a yield of 1.0 ton of alumina from 2.2 tons of bauxite (combined process) and an alumina: aluminum factor of 1.91, and a recovery of 13.5 lbs..of double salt per ton of bauxite processed, the double salt recovered from Arkansas bauxite will be equivalent to- 56.7 lbs. per ton of aluminum produced. This amount of double salt will yield about 62.4 lbs. of cryolite. Since cryolite consumption in the electrolytic aluminum process varies from 47.0 to 60.0 lbs. per ton of aluminum produced, this process provides an ideal materials balance. By the recovery of the double salt in Arkansas bauxite ore and conversion to cryolite by the process of this invention sufiicient cryolite can be obtained to provide all of the make-up in the electrolytic cells converting the alumina obtained from said bauxite ore to. aluminum.

While the preferred raw material for the process of this invention is the double salt of composition Na SO 'NaF obtained during the concentration of the spent liquors from the recovery of alumina from the sodium aluminate leach liquors of the soda-lime sintering of Bayer process red mud, it is obvious and understood that a mixture of equimolar amounts of sodium fluoride and sodium sulfate (e.g. salt cake, Glauber's salts), or a synthetically prepared co-precipitate of (b) separating the precipitate of cryolite from 'the" solution of boric acid and sulfuric acid;

of fluoboric acid-: 3 withsxfour moles "of y a double salt of composition Ha SQ -NaF and four m9le equivalentsof a member of the group consisting of aluminum hydroxide aluminum-oxide;

(IE1) separating the precipitate of cryblite fiom' tlie solution of boric acidand sulfuric acid;

(c)- reacting said solution of boric acidv and sulfuric.

acid with a calcium fluoride-containing material con taining minor amounts of calcium carbonate in quantity sufiicient to provide ten moles of reactive calciumfluoride and additional sulfuric acid in quantity suificient'to;'provide six moles of sulfuric acid in excess of the amount required to react with the calcium carbonate in the cal-* 'cium' fluoride-containing material, to form a solution of five moles of fluoboric acid and a precipitate of calcium sulfate; and v (d) separating the calcium sulfate from the solution of five moles of fluoboric acid and returningsaid solution of fluoboric acid to step (a) of the process. 1 r

3. A process for the manufacture of cryolite'which comprises the steps of: I

(a) reacting an aqueous solution containing fluoboric acid with a double salt ofcomposition Na SO -NaF and a member of the group consisting of aluminum thy droxide and aluminum oxide, said double salt being derived from the spent sodium aluminate leach liquors of thesoda-lime .sintering of the red mud desilication residue of the Bayer alumina process;

(b) separating the precipitate of cryolite from the solution of boric acid and sulfuric acid;

(c) reacting said solution of boric acid and sulfuricacid with a calcium fluoride-containing material and ad-- ditional sulfuric acid' in quantity sufficierittoforr'n an s a-s I dditi nalgsiilfuric acid'inlquantity sufficien Q6 A processfor the, manufacture pc q pr i ses ,the steps of: n n s I.

actingv an aqueous solution containing. fluobor c a withzadouble salt.ofcompositiqnjlia s I a" member of th' gr up Consisting of a umin hydr dq j and, aluminum oxide; I a

, separatingl the precipitate, of ',o'f boric acid and'sulfur ""ci acting said solution 'of' bori 1th a calciurn fluoride} ""01 te from the containm m pla ed in step ta and a" precipitate or calciumfjsulfate acid-stosteptq) oftheprocess.

a aF nd (b) separating the precipitate ofcryolite from" the solutionlof boric acid and sulfuric acid;

(c) reacting said solution of boric acid and sulfuric acid with fluorspar and additionalsulfuric acid in quan- 'tity sufiicient to form an amount of fluoboric acid equivalent tothe quantity employed in step (d) and a precipitate of calcium sulfate; and

(d) separating the calcium sulfate from the solution .of fluoboric acid and returning said solution of fluoboric acid to step (a) ofthe process; I a v 7. A process for the manufacture of cryolite which comprises the steps of; e (a) reacting an aqueous solution containing fluoboric acid with a double salt of composition Na SO -NaF and a member of the group consisting of aluminum hydroxide and aluminum oxide; t (b) separating the precipitate of cryolite from the solutionof boric acid andsulfuric acid; I

I. (c), "reacting said solution of boric acid and sulfuric acid' with fluorspar highin'silica content and additional sulfuric acid in quantity sufiicient to form an amount of fluoboric acid equivalent to the quantity employed in step (a) and a precipitateiof calcium sulfate; and

I (d) separating the calcium sulfate from the solution 1 v of fluoboric acid and "returning said solution of fluoboric amount of fluoboric acid equivalent to the quantity cm ployed in step (a) and a precipitate of calcium sulfate;

(d) separating the calcium sulfate from the solution of 1 fluoboric acid and returning said solution of fluoboric acid to step (a) of the process. a v

4. A process for the manufacture of cryolite whichv comprises the steps of:

(a) reacting an aqueous solution containing fluoboric acid with a double salt of composition Na SO -NaF and a member of the group consisting of aluminum" hydroxide and aluminum oxide, said fluoboric acid solution containing free boric acid. l

(b) separating the precipitate of cryolite from the 7 solution of boric acid and sulfuric acid;

(0) reacting said solution of boric acid and sulfuric acid with 'a calcium fluoride-containing material and additional sulfuric acid in quantity sufiicient to form an amount of fluoboric acid equivalent tothe quantity em played in step (a) and a precipitate of calcium sulfate; (d) separating the calcium sulfate from thevsolution of fluoboric acid and returning said solution of fluoboric acid to step (a) of the process.

5. A process for the manufacture of cryolite which comprises the steps of:

(a) reacting an aqueous. solution containingi fluoboric acid with a double salt of composition Na SO4-Na'F and acid to step (a)-of the process. v

8. A process for the manufacture of cryolite which comprises the steps'of:

(a)reacting an aqueous solution containing five moles of fluoboric acid vwith four moles of a double salt of composition Na SO "NaF and four mole-equivalents, of

5d a member of the group consisting of aluminum hydroxide and aluminum oxide;

b) separatingthe precipitate of cryolite from the solu- 'tion of boric acid'and sulfuric acid;

(c) reacting said solution of boric acid and sulfuric acid with a calcium fluoride-containing material containing minor amounts of calcium carbonate in quantity sufiicient'to provide l0.0molesof reactive-Cal said solution of boric acid and sulfuric acid being in the form of an aqueous medium containing from 5.0 to 7.5 moles of boric acid and sulficient sulfuric. acid to provide 10.0 moles of H 80 in excess of that quantity required to react with the CaCO-g in the calcium fluoride-containing material; to form a solution of five moles of fluoboric acid and a precipitate of'calcium sulfate;

'of five moles of fluoboric acid and returning said solution a of fluoboric acid to step j (a) of-the process.

' 9.lA process for the manufacture of cryolite which (d) separating thecalciurn sulfate from the solution comprises the steps of:' a

a member of the group consisting ofaluminum hydroxide and aluminum oxide in an" aqueous medium at a temper:

ature between C. and C. 3

, dro xideiand aluminum oxide; l

(b) separating the precipitate of cryolitefrom the 591w .tion of boric acid andsulfuric acid; a

(a) reacting an aqueous solution containing fluoboric acid with a double salt of composition Na SO -NaF and a member'ofjthe group consisting of. aluminum b l I the calcium sulfate 'frornfljthe' ot'flii boric acid and returningl said solutionfof llubbojricf (c) reacting said solution of'boric acid and sulfuric acid with a calcium fluoride-containing material and ad-' ditional sulfuric acid in quantity suflicient to form an amount of fiuoboric acid equivalent to the quantity employed in step (a) and a precipitate of calcium sulfate;

-(a) reacting an aqueous solution containing fluoboric' acid with an equimolecular mixture of sodium sulfate and sodium fluoride and a member of the group consisting 15 1,511,560

of aluminum hydroxide and aluminum oxide;

I (b)- separating the precipitate of cryolite from the solution of boric acid and sulfuric acid;

(c) reacting said solution ofhoricacid and sulfuric acid with a calcium fluoride-containing material and additional sulfuric acid in quantity sufiic'ient to form an amount of fluoboric acid equivalent to the quantity employed in step (a) and a precipitate of calcium sulfate; and

(d) separating the calcium sulfate from thesolution of fiuoboric acid and returning said solution of fiuoboric acid to step (a),of the process.

References Cited in the file of this patent UNITED STATES PATENTS Howard Oct. 14, 1924 2,182,510

Heiser Dec. 5, 1939 

1. A PROCESS FOR THE MANUFACTURE OF CRYOLITE WHICH COMPRISES THE STEPS OF: (A) REACTING AN AQUEOUS SOLUTION CONTAINING FLUOBORIC ACID WITH A DOUBLE SALT OF COMPOSITION NA2SO4.NAF AND A MEMBER OF THE GROUP CONSISTING OF ALUMINUM HYDROXIDE AND ALUMINUM OXIDE, (B) SEPARATING THE PRECIPITATE OF CRYOLITE FROM THE SOLUTION OF BORIC ACID AND SULFURIC ACIDS, (C) REACTING SAID SOLUTION OF BORIC ACID AND SULFURIC ACID WITH A CALCIUM FLUORIDE-CONTAINING MATERIAL AND ADDITIONAL SULFURIC ACID IN QUANTITY SUFFICIENT TO FORM AN AMOUNT OF FLUOBORIC ACID EQUIVALENT TO THE QUANTITY EMPLOYED IN STEP (A) AND A PRECIPITATE OF CALCIUM SULFATE, AND (D) SEPARATING THE CALCIUM SULFATE FROM THE SOLUTION OF FLUOBORIC ACID AND RETURNING SAID SOLUTION OF FLUOBORIC ACID TO STEP (A) AND A PROCESS. 